Knee braces are varied in design. Most have one component strapped to the thigh and another component strapped to the calf with a pivotable link therebetween. See U.S. Pat. No. 4,372,298 incorporated herein by this reference.
Other designs have a damper between two pivoting components and even sensors for controlling the damper. See U.S. Pat. No. 7,507,215 incorporated herein by this reference.
Many knee braces are designed with rehabilitation in mind. Typically, natural knee joint motion is inhibited or constrained in some way.
There is a different need, however, for new knee and other joint braces which can be used for other purposes. In one example, a soldier wearing a pack might benefit from a knee brace which not only provides stability but also reduces the load on the muscles of the knee, particularly the quadriceps. Indeed, when the knee bends, it would be beneficial to lower the load carried by the knee. Energy stored during the bending action can be used to straighten the knee. When the leg swings forward for the next step, it would be advantageous if the knee brace freely operated.
Most exoskeletons to date have been designed to off-load the weight of a backpack or other payload using a parallel load path of structural limbs, joints and actuators. These devices are often heavy, bulky and awkward to use.
A lighter and less-restrictive approach is to use a brace to apply torque to the limbs adjacent to a joint to assist the flexion or extension of the joint. This reduces muscle fatigue as well as reducing the skeletal forces needed to react the muscle forces. This type of assistive brace is relatively rare.
Most orthotic braces are designed to stabilize an injured joint, not apply torque. In general, the goal is to approximate the motion of the joint while protecting it from side loads and twisting.
To date, most assistive braces are adaptations of orthotic braces. As a consequence, these devices are needlessly heavy, bulky and restricting, and they typically require custom-fitting to each user.
Key research examples of assistive knee braces are as follows and incorporated herein by this reference:                Ryan U.S. Pat. No. 7,507,215 describes a knee brace with an X-style polycentric hinge, rigid upper and lower leg cuffs, and adjustable hydraulic damping.        Donelan et al, US 2013/0038056 A1 describes a knee brace with a simple hinge joint and rigid upper and lower shells. The brace is coupled to a generator that charges batteries only when the knee is doing negative work.        Grant Elliot's PhD thesis, MIT 2012, describes a knee brace with a clutchable spring. His brace uses rigid upper and lower arms and a gear-style one degree of freedom (DOF) polycentric hinge.        Yobotics Inc.'s RoboKnee uses an electric series-elastic actuator to drive a knee brace with a geared 1-DOF polycentric joint and rigid upper and lower arms.        
See also U.S. Pat. Nos. 6,971,996; 5,472,412; 3,976,057; 6,080,123; 6,981,957; 3,969,773 and US 2004/0225245 all incorporated herein by this reference.
Most braces use 1-DOF hinges and are thus very sensitive to alignment of the brace to the knee joint. This problem is much worse for assistive (torque-producing) braces. Mismatch between the instantaneous center of rotation of the hinge and of the knee causes uncomfortable or dangerous loads on the knee as well as making the brace migrate down the leg. The mismatch is inevitable because the 1-DOF hinges are only an approximation of the motion of the knee and because of deflection of the flesh where the brace attaches to the leg.
A second disadvantage with the most braces is that they require rigid structure spanning between the medial and lateral hinges. This enforces a fixed width and makes the brace wider than necessary since it must accommodate the width of the knee at the desired maximum flexion position. To avoid a sloppy fit and interference of the braces of the left and right legs, these braces will typically require custom fitting to each user.
A third disadvantage is that most prior art braces do not allow twisting of the foreleg and thus restrict a normal DOF of the leg. This makes the braces awkward to use and increases the likelihood of chafing and migration of the brace. Some knee braces are complex, difficult and expensive to mass produce, and uncomfortable to use in daily activities.